Elsevier

Quaternary Science Reviews

Volume 78, 15 October 2013, Pages 248-265
Quaternary Science Reviews

Micro-investigation of EPICA Dome C bottom ice: evidence of long term in situ processes involving acid–salt interactions, mineral dust, and organic matter

https://doi.org/10.1016/j.quascirev.2013.08.012Get rights and content

Highlights

  • EPICA Dome C bottom ice, 75–15 m above the bedrock, older than 800 ka.

  • Ion chromatography, synchrotron X-Ray micro-fluorescence, electron microscopy.

  • Ice recrystallization, acid–aerosol interactions, bacterial reactivation.

  • Eolian input: biogenic debris and mineral dust from un-iced peri-Antarctic areas.

  • Local input and subglacial environment.

Abstract

The EPICA Dome C ice core (EDC) reached a final depth of 3260 m, at a maximum height of about 15 m above the ice–bedrock interface in December 2004. We present here data gained from a detailed investigation of selected samples of the deeper part of the core located below 3200 m and referred to as bottom ice. This part of the core has been poorly investigated so far mainly because there are significant challenges in interpreting paleo-records that were very likely modified by long term in situ processes. Our study combines high resolution ion chromatography, high resolution synchrotron X-Ray micro-fluorescence (micro XRF), scanning, and transmission electron microscopy. Our aim was to identify the long term physico-chemical processes at work close to the bedrock, to determine how they have altered the initial registers, and, ultimately to extract information on the very ancient Antarctic environment.

The ubiquitous presence of nanometer iron oxide crystals at the surface of wind-borne dust aggregates containing also large amount of organic matter raises the possibility that the consolidation of windborne dust clusters formed during ice recrystallization could be related to microbial iron reduction and, thus, to the progressive reactivation of dormant bacterial activity in warming ice. Inclusions of size and number density increasing with depth observed in the 12 last meters (3248–3260 m) contain liquid and solid species, among them marine biogenic acids, numerous wind-borne dust aggregates and clusters of large reversible calcium carbonate particles precipitated once the inclusion was formed and often covered by secondary gypsum. The refreezing of slush lenses is discussed as a potential cause of the formation of such heterogeneous and complex mixtures. In addition to the very fine micrometer size minerals windborne from extra-Antarctic continental sources and often accreted in large aggregates, single medium size particles (a few to ca 20 μm and among them organic debris) are commonly encountered. Their size, surface shape, and mineralogy suggest that aerosol transport from Antarctic ice-free areas played a significant role at the time EDC bottom ice was formed. Concentrations and concentration ratios of biogenic sulfur species also advocate for the strengthening of peri-Antarctic meteorological patterns that favor the inland penetration of disturbed flow carrying local material. Very large well preserved mineral particles several tens of micrometers in diameter, and biotope relics in deeper ice close to 3260 m likely come from the sub-glacial environment.

Introduction

Deep ice cores recovered in central Greenland and on Antarctic sites are considered to be relevant archives of past changes in the climate and the atmosphere's composition for time periods ranging from the last climatic cycle to several hundred thousand years back. While a reliable time–depth relationship is essential to the interpretation of proxy records in terms of paleo-environmental history, ice dating becomes quite uncertain in the lowermost sections of ice cores where time series can be disrupted or altered by flowing ice or by the thermal regime due to the bedrock's proximity. However, and despite the lack of chronology, the exhaustive study of impurities gathered in the deepest layers of polar ice, i.e. close to the ice sheet base and generally referred to as bottom or basal ice, can give innovative clues for understanding the very ancient polar environment and sub-glacial properties. Moreover, closed or open high pressure–high temperature systems encountered at the bottom of polar ice caps can provide a useful illustration for the study of very long term physico-chemical and possibly biological processes in relation with initial ice content and grain growth conditions.

  • Information available from previous deep drilling projects

The few studies conducted to date on Greenland and Antarctic bottom ice provide interesting insights on pre-glacial environments and in-situ processes. These are summarized below.

Dielectric profiling (DEP) measurements providing ice conductance and capacitance at a range of frequencies that span the main dielectric dispersion of ice were performed in the 6 m of the basal silty ice sequence from the GRIP ice core (Summit, Central Greenland) and the detailed profile of the high frequency limit of the conductivity (σ∞), the most useful parameter that can be deduced from dielectric measurements, was compared with multi-parameter studies involving water stable isotope, debris content, gas composition of CO2 and CH4 and ion concentrations (Tison et al., 1998). These authors concluded that (σ∞) was fully explained by the intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice, which may have been initially present as gaseous NH3. Whilst ammonium peaks in the higher part of the ice core are related to the deposition of biomass burning products, and among them ammonium formate (Legrand and de Angelis, 1995, Legrand and de Angelis, 1996), carboxylates are largely dominated by oxalate in the basal sequence. The strong correlation between oxalate, ammonium and calcium and the net excess of oxalate compared with what could be expected from uric acid degradation alone led the authors to propose that this basal sequence results from the incorporation of a “local end-term” (firn, permafrost ice) involving local biogenic production by plants and animals that was formed in the absence of the present-day ice sheet.

The first comprehensive study of the ionic composition of accretion ice of Lake Vostok, the largest Antarctic sub-glacial lake, combined with additional isotopic and iron measurements suggested that sedimentary sequences with a composition close to evaporite contribute to the lake chemistry (De Angelis et al., 2004). The second step was to investigate accretion ice using high resolution synchrotron X-Ray micro-fluorescence (De Angelis et al., 2005). Liquid brine micro-droplets (3–10 μm) were observed, that coexist with large irregular sulfur-rich aggregates (10–800 μm) containing gases and a mixture of very fine particles. Most of these objects were sequestered inside large ice crystals that grew slowly after ice formation. Their structure and composition provides evidence of hydrothermal activity at the lake bottom and of haline water pulses carrying fine solid debris perhaps biota from a deeper evaporitic reservoir into the lake. The presence of both reduced and oxidized sulfur forms tightly associated in solid inclusions was particularly interesting regarding potential bacterial activity. The coexistence in ice lattice of relatively scarce large inclusions with regularly scattered micro-droplets can be explained by relocation processes: at high temperature (−3 °C for accreted ice, under in situ conditions), the minimization of grain boundary free energy induces abnormal grain growth, leading to grain boundaries of high crystalline quality (Montagnat et al., 2001). At the beginning of the grain growth process, solid particles initially homogeneously distributed in the ice lattice are probably caught up in moving grain boundaries, in order to reduce free surface energy. However, particles aggregate with progressive grain growth and, when large enough, the aggregates remain in the ice lattice. Considering its high salinity, the brine is likely in a liquid state at −3 °C. Interaction between grain boundaries and liquid inclusions is probably similar to the interaction between ice and water, which means that brine bubbles can remain in the ice lattice.

  • The EPICA Dome C core:

The EPICA Dome C ice core (EDC) is one of the two ice cores drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA). The drilling site is located on the East Antarctic Plateau at Concordia Station (75°06′04″S; 123°20′52″ E, 3233 m above sea level), about 1200 km inland. The core reached a final depth of 3260 m estimated to be about 15 m or less above the ice–bedrock interface, based on seismic sounding in the drill hole (Schwander, personal communication). Detailed information on bedrock and surface topography were gained by altimeter and airborne radar surveys conducted in 1994–1995 (Rémy and Tabacco, 2000) and from 1995 to 2001 (Forieri et al., 2004). The most notable bedrock features in the 50 km × 50 km enlargement of the Dome C central area include sub-glacial highlands, a set of north–south-trending parallel valleys tens to hundreds of meters deep and a few cirque bowls. The detailed description of the bedrock under the drilling site provided by Forieri et al. (2004), highlights a topographic depression surrounded by hills on the order of 50–100 m high. Rémy and Tabacco, 2000, and Forieri et al., 2004, concluded that the bedrock topography below the Dome C area developed before the East Antarctic ice cap and was not significantly modified by subsequent glacial erosion, mountain ridges being very likely of tectonic origin while the valleys may have been formed by erosion by wet-based mountain glaciers, weathering of granitic rocks, or karstification of limestone.

While high resolution multi-parameter analyses performed along the upper 3140 m of the core have provided a wealth of paleo-environmental data over the last 8 climatic cycles (see for instance EPICA_Community_members, 2004, EPICA Community Members, 2006, Loulergue et al., 2008, Delmonte et al., 2008, Lüthi et al., 2008, Kaufmann et al., 2010, Wolff et al., 2006, Wolff et al., 2010), the deeper part of the core has remained poorly investigated. The ionic content associated with very sharp sulfate spikes observed between 2800 and 3140 m, was determined by Traversi et al. (2009). Compared with volcanic events recorded in the upper part of the core, these spikes seemed anomalous, with unusually low acidity, high Mg2+ concentrations, high Mg2+/Ca2+ ratios, and significant Mg2+–SO42− correlation. The authors suggest that long term rearrangement of impurities via migration in the vein network led to the formation of soluble magnesium sulfate particles in liquid film at grain boundaries. The dust size profile is not available below 2900 m because of the presence of particle aggregates of unknown origin (Lambert et al., 2008). The first data gained between 3200 and 3260 m reported in Jouzel et al. (2007), show a much lower than expected signal variability of water isotope and deuterium excess records, shared by the oxygen 18 of O2 in air and preliminary dust mass, CH4, and CO2 data, that are however in the range of concentrations found in ice formed under full glacial conditions. Air content comparable to shallower values led these authors to dismiss large scale melting and refreezing as a plausible explanation of the profiles they observed and to suspect that this part of the core has been affected by flow disturbances due to stretching of the ice sequence or mixing of layers of different origins. Based on radiometric ages provided by (234U/238U) activity ratios for a set of samples taken along the EDC core, Aciego et al., 2011, observed a marked change below 3200 m, i.e. in the deepest potentially disturbed ice sequence, with ages ranging from 870 ka, the oldest age measured in the upper part of the sequence, to 85 ka in the deepest sample. They concluded to homogenization of the deep ice prior to resetting of the (234U/238U) in the deepest sample and proposed that the low chemical variability results from vertical ice stretching due to sub-glacial melting and enhanced lateral ice flow while the age resetting in basal ice layers was explained by melting, recrystallization and precipitation processes. They considered as likely that this part of the core consists of ice that spans a relatively short age interval corresponding to the cold stage 20.2. The ice of the last 12 m (i.e. from 3248 to 3260 m) contains visible rounded shaped inclusions, brownish to reddish in color, and increasing in size (from less than 1 mm up to a few mm in diameter) and number density (from less than 10 to more than 20 inclusions per ice section 50 cm long) with depth. These inclusions are generally located at grain boundaries or triple junctions (Tison et al., 2013). Following these authors, this part of the core will be referred to as “basal” ice, from 3200 to 3248 m the ice will be referred to as “deep” ice, and we will use the term “bottom” ice to qualify the whole ice sequence below 3200 m.

In this paper, we present and discuss data gained along the bottom part of the EDC core through a combined analytical approach involving ion chromatography (IC), high resolution synchrotron X-Ray micro-fluorescence (micro-XRF) developed using the European Synchrotron Radiation Facilities (ESRF), and scanning (SEM) and transmission (TEM) electron microscopy. The location of the full set of samples along the bottom ice sequence is summarized in the upper part of Fig. 1. In their companion study (submitted), Tison et al. use part of the IC data in a quite different and complementary way focusing on a systematic comparison of the concentration mean values and distribution frequencies of selected markers in bottom ice lamellae and every previous full glacial episode (i.e. depicting similar water stable isotope ranges) to determine whether a clear paleoclimatic signal can be retrieved in the deeper part of the EDC core. They conclude to a fairly good preservation of the signal in terms of global ice properties but that the time scale has been considerably distorted by chemical stretching due to the increasing influence of sub-glacial topography. We focus here on relocation processes and their link with bio-chemical interactions likely to lead to the formation of large aggregates of soil dust particles, inclusions, and secondary minerals. A careful examination of the full data set makes it possible to highlight some particularities of the bottom ice chemical imprint and provides also information on the very ancient Antarctic environment.

Section snippets

Ion chromatography

IC was applied first to a preliminary set of 9 discontinuous samples and then to high resolution sampling of 10 ice lamellae 25–50 cm long. The 9 samples of the preliminary study were initially ca 8 cm long, 7 of them were taken in deep ice, 2 in basal ice. They were rinsed in 3 successive baths of ultrapure water, a decontamination procedure proved to be efficient even when measuring organic traces in Antarctic ice cores extracted within drilling fluid (De Angelis et al., 2012). However and

In situ processes

The interpretation of species ultimately identified in ice archives requires that the relative influences of processes related to firn/ice aging are disentangled from those occurring in the atmosphere. This is a difficult task not only because these processes are complex but also because similar chemical compounds may be produced through chemical atmospheric interactions as well as in situ interactions.

As a general rule, heterogeneous reactions occurring during transport between the water

Past Antarctic environment and the role of Antarctic sources

Very fine micrometer or sub-micrometer sized alumino-silicate particles were ubiquitous in all bottom ice samples, among them clay, feldspar, quartz, and muscovite particles as commonly found in Antarctic ice cores. This is consistent with a long-range input from Southern Hemisphere continents dominated by Patagonian sources (Delmonte et al., 2010). However, several features that may be considered specific to bottom ice were revealed by ion chromatography measurements and SEM and TEM

Conclusion

Four major findings emerge from the multi-technique approach presented here:

Not only time but above all ice temperature plays a key role in two long term processes taking place in the deepest part of the EDC core. The first is the interaction between acidic species in grain boundaries and alkaline aerosol present in ice crystals and the second is the consolidation of the large wind-borne dust aggregates progressively formed through grain boundary migration, for which we propose here a

Acknowledgments

This work is a contribution to EPICA, a joint European Science Foundation (ESF)/European Commission (EC) scientific program, funded by the EC (EPICA/MIS) and by national contributions from Belgium, Denmark, France, Germany, Italy, the Nederland's, Norway, Sweden, Switzerland and the UK. We acknowledge technical support from the C2FN (French National Center for Coring and Drilling), handled by INSU. We thank all the personnel who have contributed to obtain ice core sampling. We thank Paul Duval

References (91)

  • J.N. Perdrial et al.

    Interaction between smectite and bacteria: Implications for bentonite as backfill material in the deposal of nuclear waste

    Chem. Geol.

    (2009)
  • K. Pol et al.

    New MIS 19 EPICA Dome C high resolution deuterium data: hints for a problematic preservation of climate variability at sub-millenial scale in the “oldest ice”

    Earth Planetary Sci. Lett.

    (2010)
  • B. Sohlenius et al.

    Occurrence of nematodes, tardigrades and rotifers on ice-free areas in East Antarctica

    Pedobiologia

    (2004)
  • R. Udisti et al.

    Sea spray aerosol in central Antarctica. Present atmospheric behavior and implications for paleoclimatic reconstructions

    Atmos. Environ.

    (2012)
  • E.W. Wolff et al.

    Changes in environment over the last 800,000 years from chemical analysis of the epoch Dome C ice core

    Quat. Sci. Rev.

    (2010)
  • P. Artaxo et al.

    Trace element and individual particle analysis of atmospheric aerosols from the Antarctic Peninsula

    Tellus B

    (1992)
  • L. Augustin et al.

    Epoch Dome C drilling operations: performances, difficulties, results

    Ann. Glaciol.

    (2007)
  • O. Braissant et al.

    Exopolymeric substances of sulfate-reducing bacteria: interactions with calcium at alkaline pH and implication for formation of carbonate minerals

    Geobiology

    (2007)
  • S.A. Bulat et al.

    Cell concentrations of microorganisms in glacial and lake ice of the Vostok ice core, East Antarctica

    Microbiology

    (2009)
  • F.H. Chapelle et al.

    Competitive exclusion of sulfate reduction by Fe(III)-reducing bacteria: a mechanism for producing discrete zones of high-iron ground water

    Ground Water

    (1992)
  • M. De Angelis et al.

    Long term trends of mono-carboxylic acids in Antarctica: comparison of changes in sources and transport processes at the two EPICA deep drilling sites

    Tellus B

    (2012)
  • M. De Angelis et al.

    Brine micro-droplets and solid inclusions in accreted ice from Lake Vostok (East Antarctica)

    Geophys. Res. Lett.

    (2005)
  • M. De Angelis et al.

    Volcanic eruptions recorded in the Illimani ice core (Bolivia): 1918–98 and Tambora periods

    Atmos. Chem. Phys.

    (2003)
  • M. De Angelis et al.

    Origins and variations of fluoride in Greenland precipitation

    J. Geophys. Res.

    (1994)
  • M. De Angelis et al.

    Sources of continental dust over Antarctica during the last glacial cycle

    J. Atmos. Chem.

    (1992)
  • P. De Deccker et al.

    Geochimical and microbiological fingerprinting of airborne dust that fell in Canberra, Australia, in October 2002

    Geochem. Geophys. Geosys.

    (2008)
  • B. Delmonte et al.

    Aeolian dust in East Antarctica (EPICA-Dome C and Vostok): Provenance during glacial ages over the last 800 kyr

    Geophys. Res. Lett.

    (2008)
  • A. De Los Rios et al.

    Ultrastructural and genetic characteristics of endolithic cyanobacterial biofilmscolonizing Antarctic granite rocks, FEMS Microbiol.

    Ecol.

    (2007)
  • G.S. Dieckmann et al.

    Calcium carbonate as ikaite crystals in Antarctic sea ice

    Geophys. Res. Lett.

    (2008)
  • G.S. Dieckmann et al.

    Brief communication: Ikaite (CaCO3.6H2O) discovered in Arctic sea ice

    The Cryosphere.

    (2010)
  • G. Durand et al.

    Evolution of the texture along the EPICA Dome C ice core

  • EPICA_Community_Members

    Eight glacial cycles from an Antarctic ice core

    Nature

    (2004)
  • EPICA Community Members

    One to one coupling of glacial climate variability in Greenland and Antarctica

    Nature

    (2006)
  • A. Forieri et al.

    New Bedrock map of Dome C, Antractica, and morphostructural interpretation of the area

    Ann. Glaciol.

    (2004)
  • H. Gallée

    Mesoscale atmospheric circulations over the Southwestern Ross Sea Sector, Antarctica

    J. Appl. Meteor.

    (1996)
  • A. Gaudichet et al.

    An investigation by analytical transmission electron microscopy of individual insoluble microparticles from Antarctic (Dome C) ice core samples

    Tellus

    (1986)
  • A. Gaudichet et al.

    Mineralogy of insoluble particles in the Vostok Antarctic ice core over the last climatic cycle (150 kyr)

    Geophys. Res. Lett.

    (1988)
  • D. Gebauer et al.

    Stable prenucleation calcium carbonate clusters

    Science

    (2008)
  • F.E. Genceli Guner et al.

    Crystallization and characterization of magnesium methanesulfonate hydrate Mg(CH3SO3)2.12H2O

    Crystal Growth Des.

    (2010)
  • D.W. Griffin et al.

    Atmospheric microbiology in the northern Caribbean during African dust events

    Aerobiology

    (2003)
  • K. Hara et al.

    Variations of constituents of individual sea-salt particles at Syowa station, Antarctica

    Tellus

    (2005)
  • S. Harder et al.

    Sulfate in air and snow at the South Pole: implications for transport and deposition at sites with low snow accumulation

    J. Geophys. Res.

    (2000)
  • C. Holz et al.

    Terrigenous sedimentation processes along the continental margin off NW-Africa: implications from grain size analyses of surface sediments

    Sedimentology

    (2004)
  • Y. Iizuka et al.

    A relationship between ion balance and the chemical compounds of salt inclusions found in the Greenland Ice Core Project and Dome Fuji ice cores

    J. Geophys. Res.

    (2008)
  • Y. Iizuka et al.

    Sulphate-climate coupling over the past 300,000 years in inland Antarctica

    Nature

    (2012)
  • Cited by (16)

    View all citing articles on Scopus
    View full text